I don’t know that there is an exact dictionary definition of banners and pennants used in the sports marketplace, but here’s the best explanation I can offer.
1. A Banner is a large/long piece of cloth or fabric with a design, picture, or writing on it. For our purposes, I’d go further and say that a banner is a rectangular-ish shaped cloth that is generally longer than it is wide. That’s as opposed to a flag which is generally wider than it is tall. So think of a banner as a flag turned 90 degrees, ie a flag that hangs down. I say “rectangular-ish” because some of the banners shown on this site are rectangular, while others have shapes that aren’t true rectangles but are roughly rectangular (thus the –ish).
2. Pennants are easier to describe – they are fabric/cloth that is triangular in shape with a design, picture or writing on it. As the dictionary says, a pennant is “any relatively long, tapering flag”. It doesn’t matter if the pennant is horizontal or vertical in design, it’s still a pennant. In sports we generally think of pennants as being horizontal. We also tend to think of pennants as being associated with baseball, where a pennant is described as a flag serving as an emblem of victory or championship. But nowhere is it written that a pennant has to be horizontal, only that it be triangular.
In case you like trivia, this site has roughly 850 products from Winning Streak Sports. Of the 850 products:
A. 75 are metal tavern signs – 9%
B. 620 are banners – 73%
C. 150 are horizontal pennants - 17%
D. 10 are vertical pennants - 1%
Thus when I named this website “Heritage Sports Banners”, I did so with the full knowledge that it isn’t an entirely accurate name – forgive me – at least I’m 73% right!